1. Field of the Invention
The present invention relates to the single-stage preparation of a hydroxyl-containing aromatic compound by selective catalytic oxidation of an aromatic hydrocarbon.
2. Discussion of the Related Art
To prepare a hydroxyl-containing aromatic compound, such as phenol, it has not been possible hitherto to transform a benzene directly into the corresponding hydroxyl compound by selective oxidation in a single stage and in high yield. Either the aromatic ring was not attacked at all by the oxidizing agent or it was destroyed by the action of the oxidizing agent. The principal products formed were carbon dioxide and coke.
From an economic point of view, it has only been possible hitherto to introduce a hydroxyl group into an aromatic system via a plurality of intermediate stages.
The cumene process has become established industrially for preparing phenols starting from benzenes. According to this process, usually, the cumene prepared from benzene and propene is peroxidized and the oxidation product is then cleaved into phenol and acetone.
In addition, a benzoic acid process starting from toluene is used, in which process the benzoic acid prepared from toluene can be decarboxylated to give phenol. The decarboxylation step, i.e. the loss of an organically bound carbon, however, consumes at this early stage any price advantage of toluene over benzene. The process is therefore only of interest if the intended product is benzoic acid and free capacities for preparing phenol are utilized.
Other processes for preparing phenol, e.g. via chlorobenzene (chlorination or oxychlorination of benzene) or the sulfonation process (preparation of benzenesulfonic acid) have proved to be uneconomic. Reasons for this were in part the unsatisfactory selectivity, corrosion problems and the production of unwanted by-products.
The cyclohexanol process (hydration of cyclohexene in the 1 st stage) is also uneconomical. The process goes through too many stages to arrive at the target product phenol.
For this reason, internationally, the majority of phenol is prepared via the above-mentioned cumene route. However, since in this process acetone is also produced, the cost-effectiveness of this process is dependent on the market prices for phenol and acetone.
In order to bypass the dependence on the coupled product acetone, many attempts concentrate on the selective oxidation of benzene or benzene derivatives. Thus, for example, U.S. Pat. Nos. 5,055,623, 5,672,777 and 5,110,995 describe the oxidation of benzene by dinitrogen monoxide on suitable catalysts.
The oxidizability of benzene by dinitrogen monoxide on vanadium 20 oxide/molybdenum oxide/tungsten oxide catalysts has already been known since 1982 (Iwamoto). Zeolite catalysts of the ZSM-5 type were discovered for the direct oxidation of benzene by dinitrogen monoxide in 1988 (Gubelmann, Rhone-Poulenc). The mode of action of zeolites is fundamentally due to their microporous channel system having pore diameters of the size of the molecules to be oxidized.
The use of iron-containing zeolite catalysts of the ZSM-5 type for oxidizing benzene by dinitrogen monoxide is described by Volodin, Bolshov and Panov in J. Phys. Chem. 1994, 98, 7548-7550.
The following documents also describe the oxidation on zeolites, in particular of the ZSM-5 type: 3rd World Congress on Oxidation, 1997 Elsevier Science, B. V., R. K Graselli et al., (Editors), M. Hafele et al. (University of Erlangen-Nuremberg) and G. I. Panov et al., Applied Catalysis A: General 98 (1993) 1-20.
The reaction which is most developed in terms of process engineering is that of dinitrogen monoxide on acid zeolites of the ZSM-5 and ZSM-11 type with various metal additions (e.g. iron). The reaction is usually carried out at atmospheric pressure and temperatures of 300-450.degree. C.
It is a disadvantage of zeolites that because of their completely crystalline structure, the variation in pore size cannot be adapted continuously to the molecule to be oxidized, but is only possible stepwise, dependent on the crystal type which is established. This means that zeolites cannot be specifically adapted to the respective oxidation problem. The surface polarity, the setting of which makes certain reaction mechanisms conceivable, is also virtually invariable for zeolites. The literature, such as U.S. Pat. Nos. 5,110,995 or 5,055,623 describes one such zeolite type (the pentasils ZSM-5 and ZSM- I1) for preparing many phenol derivatives. Although zeolites or acidic zeolites can be modified by various metals, this can at best improve, but not completely solve the problem described above. Thus, for example, in an acid zeolite, a hydrogen atom can be exchanged for a sodium atom. In the iron-containing zeolite, in this case, no loss of activity was observed; whereas, when the iron is replaced by aluminum, the activity is decreased when a hydrogen atom is exchanged for a sodium atom. Overall, however, conversion rate and selectivity are still unsatisfactory.
Although changing other parameters, such as higher reaction temperature (zeolites are considered heat-stable up to about 800.degree. C.) leads to higher benzene conversion rates, it also leads to lower selectivity and greater deactivation of the catalyst. Increasing the partial pressure of N.sub.2 O can also increase the benzene conversion rate but this is likewise at the cost of selectivity. A gradual improvement can, at best, be achieved by increasing the partial pressure of benzene. The selectivity and the resulting amounts of phenol increase somewhat. Nevertheless, it is still desirable to increase the selectivity and degree of conversion further.
The relatively rapid coking of the catalyst, which occurs at the temperatures usually used leads to a loss of activity and the catalyst must, therefore, be regenerated relatively frequently (about every 48 hours).
The dinitrogen monoxide used for the catalytic oxidation of benzenes, in the processes described above, has to have a very high degree of purity. Contaminants such as oxygen or hydrophilic gases such as steam or ammonia can inactivate the zeolite catalyst to the point of inactivity. Only inert gases such as noble gases or nitrogen can be tolerated as admixtures.
Various sources of dinitrogen monoxide are suitable. The catalytic decomposition of ammonium nitrate at 100-160.degree. C. using manganese, copper, lead, bismuth, cobalt and nickel catalysts gives a mixture of dinitrogen monoxide, nitrogen oxide and nitrogen dioxide, so that the gas cannot be used directly for oxidizing benzene.
Somewhat more expedient is oxidizing ammonia by oxygen on platinum oxide or bismuth oxide catalysts at 200-500.degree. C. and reacting nitrogen oxide with carbon monoxide on platinum catalysts. However, in the first case, water is produced as by-product and in the second case carbon dioxide. Further, the dinitrogen monoxide prepared in this manner can not be used directly for the benzene oxidation. Similarly, the dinitrogen monoxide arising in the production of adipic acid cannot be used directly for the oxidation but must be subjected to a separate purification step. The oxygen present in the exhaust gas and the NO.sub.x interfere in particular.
The purity of the dinitrogen monoxide must also be seen against the background that zeolites absorb water, which decreases their catalytic activity. Although a few zeolite structures may be hydrophobized by dealumination, this further restricts the selection of suitable zeolites. The dealumination is, furthermore, an additional process step and leads to unwanted amorphous contents in the zeolite. In addition, the extent of the dealumination cannot be set specifically, so that with regard to the process, this must be determined empirically. This means that fluctuating quality grades of oxidizing agent, such as differing admixtures of steam, cannot be used.